• State Resolved
  • Locked Locked
  • Replies 1 reply
  • Subscribers 62 subscribers
  • Views 500 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

UCC28631: Rb value

Aaron Spaete
Expert 4290 points
Part Number: UCC28631

Team,

My customer has the following question. 

I made over look this calculation.  When calculating Rb, the magnetic sense resistor, from the datasheet as follows in section 10.2.4.12. 


If I have multiple outputs, what output am I using for Rb variable Vout?


Thanks as always,

Aaron

  • 0
    TI__Mastermind 29710 points
    Hello Aaron,

    That is a good question, which is applicable to the other primary-side regulated (PSR) controllers (UCC287xx series) besides the UCC28631 using magnetically-coupled output sensing.

    In the case of multiple outputs, more specifically: multiple output windings, the winding with the highest output power draw is the one that influences the voltage sensing on the primary-side bias winding. Therefore, the target voltage for the highest power output is used to determine the value of Rb.

    This is relatively clear when the main output power is significantly higher than any of the other outputs. Here, the main output voltage regulation will be reasonably tight, but the voltage regulation of the minor outputs can be significantly looser. In cases where several outputs have similar or identical output power levels, the regulation of any one of those outputs will not be as well controlled since the leakage inductances of each winding are unlikely to be the same. This loose-regulation issue is exacerbated if the percent-loading of these windings do not track, but change independently. It may be difficult to determine which output will be the dominant one, but in such a case the output which must deliver the highest power most often under the varying operating conditions should be chosen to calculate Rb.

    I hope this answers your question and results in a satisfactory design.

    Regards,
    Ulrich